1. Field of the Invention
The present invention relates to a text protection device for de-interlacing operation, and more particularly to, a text protection device for a motion adaptive de-interlacing operation.
2. Description of the Prior Art
An image display technology can be classified into an interlaced scan and a progressive scan. When a frame is displayed, the conventional interlaced scan technology sequentially scans two fields composed of the odd scan lines of the frame and the even scan lines of the frame, respectively. The progressive scan technology (i.e. non-interlaced) sequentially scans all scan lines of the frame by multiple horizontal scan frequency to obtain a better performance on display.
Since each frame of the interlaced image is only half an original frame, if the interlaced image intends to be displayed by the progressive scan technology, an interlaced to progressive conversion (i.e. de-interlace) must be performed on the interlaced image in order to re-generate a scan line between two adjacent scan lines of the interlaced scan frame. In the prior art, motion adaptive de-interlacing (MAD) is an advanced de-interlacing technology, which can distinguish a dynamic image area or a static image area according to pixel value differences of the fields, and selects an inter-field interpolation method (e.g. WAEVE mode) to process the static image area, selects an intra-field interpolation method (e.g. BOB mode) to process the dynamic image area, or even selects blending of the two methods according to motion magnitudes of the image area.
The inter-field interpolation uses data of adjacent fields to generate a complete frame by interpolation. When the frame is completely static, through the inter-field interpolation, it is sufficient to generate an ideal re-establishing image. Besides, the vertical resolution can be as good as that in the original non-interlaced frame. However, when there is a moving object in the frame, the inter-field interpolation method may result in flicker on the displayed image since positions of the moving object in two adjacent fields are different.
The intra-field interpolation is adapted to re-establishment of the frame with moving objects. Since the intra-field interpolation uses single field data to re-establish the whole frame, which is irrelevant to object motion between adjacent fields, the image obtained by intra-field interpolation thus has a better motion effect. In addition, since the simple vertical interpolation easily results in saw-tooth shaped images, the prior art further detects edges of the image object through an edge detection mechanism and performs the intra-field interpolation operation along edge directions of the detected edges to avoid saw-toothed artifacts appearing on the object image.
However, when the video signals have a moving text image, such as running captions in a news channel or a scrolling cast list in a film, if the moving text image is processed by the intra-field interpolation method according to the prior art, since the direction with a smaller pixel value difference is considered as the edge of the image object by the edge detection mechanism, the text image is likely suffered from uncomfortable “tipped text” artifacts on the text stroke intersection or the horizontal text stroke of the text image due to false determinations of the edge direction, as shown in FIG. 1. Consequently, the user's demand for the high quality image can not be met.